Bipolar Complementary Metal Oxide Semiconductors (BiCMOS) devices are semiconductor devices that combine bipolar and Complementary Metal Oxide Semiconductors (CMOS) on the same chip. Increasing the performance of these BiCMOS devices has been a constant effort so as to maximize the efficiency and reduce the size of the BiCMOS device.
Various methods have been used to increase the performance of BiCMOS devices. One such method involves forming a BiCMOS device with shallow trenches filled with a dielectric layer that has a lower thermal expansion coefficient than that of silicon. This inevitably results in a biaxial compressive strain, which causes a uniaxial tensile strain in the direction of collector current. However, while this biaxial compressive strain has been reported to enhance Bipolar Junction Transistors (BJTs) and Heterojunction Bipolar Transistors (HBTs), the biaxial compressive strain actually has a detrimental effect on the performance of other CMOSs on the same chip, especially for an NMOS transistor.
Another method that has been used to increase the performance of BiCMOS devices has been to use tensile or compressive stress films as contact etch stop layers (CESLs) over the devices. A compressive stress CESL, when formed over a PMOS transistor, can increase the performance of the PMOS transistor by acting to strain the channel region. However, the same compressive CESL, if formed over either an NMOS transistor or a BJT, will actually work to degrade the performance of the NMOS or BJT.
Conversely, a tensile CESL, when formed over either an NMOS transistor or a BJT, can increase the performance of the NMOS transistor or BJT by acting to strain the channel region of the devices. Unfortunately, this tensile CESL will have the opposite effect upon a PMOS transistor on the same chip, and will degrade the performance of the PMOS transistor. A single CESL placed on a chip cannot simultaneously enhance a PMOS transistor, an NMOS transistor, and a BJT.
Because of these and other problems associated with the current methods of forming BiCMOS devices, a new BiCMOS device that improves the performance of all of the devices on the chip is needed.